The determination of gas and liquid flow rates in gas-liquid fluid mixtures is important in the oil and gas industry.
An example of an apparatus for measuring such flow rates is Schlumberger's Vx™ system (see e.g. I. Atkinson, M. Berard, B.-V. Hanssen, G. Ségéral, 17th International North Sea Flow Measurement Workshop, Oslo, Norway 25-28 Oct. 1999 “New Generation Multiphase Flow meters from Schlumberger and Framo Engineering AS”) which comprises a vertically mounted Venturi flow meter, a dual energy gamma-ray holdup measuring device and associated processors. This system allows the simultaneous calculation of gas, water and oil volumetric flow rates in multi-phase flows.
In the Vx™ system, and in other conventional multiphase flow meters, it is important to know the gas density at line conditions. Thus, generally it is necessary to determine the gas pressure at line conditions. For non-swirling gas-liquid flows as in the Vx™ system, the pressures at the pipe wall and pipe centre over the same pipe cross-section are substantially the same, so the pressure measured at the pipe wall at the Venturi throat section will be substantially the gas pressure thereat.
A multiphase flow meter concept is disclosed in GB A 2447490. The concept is based on swirling the flow by centrifugal force to generate a liquid annulus and a gas core. Sensors, such as ultrasonic clamp-on transducers, are used to measure the liquid-layer properties, such as the liquid-layer thickness and the axial velocity, at a Venturi throat section. These can give a direct measurement of the liquid flow rate as a result of a controlled, annular-type flow distribution over a wide range of flow conditions. For swirling gas-liquid flows, however, the pressures at the pipe wall and pipe centre over the same pipe cross-section are markedly different, so the pressure measured at the pipe wall will not be the gas-core pressure that is needed for the gas-density calculation.